1. Field of the Invention
The present invention relates to the briquetting of mixtures of coal and coke.
More particularly, the invention relates to the hot-briquetting of such coal/coke mixtures.
2. The Prior Art
It is known from the prior art to convert coal into coke--e.g., in a fluidized-bed reactor or in a stream of hot gas--to obtain hot briquetting coke which is then mixed with coking (i.e., bituminous) coal whereupon the resulting mixture is briquetted at a temperature of about 400.degree.-500.degree. C.
The amount of coking coal which is added to form the mixture varies between 20 and 40% by weight of the mixture, depending upon the characteristics of the coal. The weight percentage must be the higher, the lower the bitumen content of the coal in order to obtain briquetts of adequate strength (i.e., resistance to crumbling, breaking and abrasion).
In the prior art the coal will have a temperature within the range of 10.degree.-100.degree. C. prior to mixing with the coke which itself will have a temperature of between about 700.degree.-800.degree. C. The coal temperature depends upon the source from which the coal is fed; a 100.degree. C. temperature will obtain if the coal is added to the coke immediately after undergoing a drying operation. Because of the temperatures of the coal and coke relative to one another, the temperature of 400.degree.-500.degree. C. desired for the coal/coke mixture can be readily attained as a result of the mixing.
However, difficulties have been experienced in obtaining briquettes of uniform quality, especially uniform strength, with the prior-art approach. Upon long and detailed investigation of the problem it was found that contrary to previous belief the quality and strength of the briquettes are dependent not merely upon the weight percentage of coal in the briquetting mixture (making allowance for the characteristics of the particular coal). The quality and strength of the briquettes are, surprisingly, also influenced--and at least equally so--by another factor, namely the briquetting temperature which must be individually accommodated (upon empirical determination) to the characteristics of the coal and the coke in a particular mixture. It has been found to be of central importance that the temperature of the mixture being briquetted not only be prevented as much as possible from fluctuating, but that it be adjusted to within substantially .+-.5.degree. C. to an individual briquetting temperature. Contrary to previously held beliefs it is not possible to obtain briquettes of consistently maximum quality and strength by adjusting the temperature to within e.g., .+-.20.degree. C. in the 400.degree.-500.degree. C. range.